Is it possible to change the boundary condition type (like wall, interior etc) during simulation? Can this be specified in the UDF? I intend to have different BC type for certain conditions during simulation.

Dear Apruv;
I come across with your post while I was searching, i think, for a similar problem. I 've seen that unfortunately you have not gotten any response to your post, nevertheless I thought that you may have reached a solution.

I am trying to cool a room with a constant temperature wall for a certain time, and than turn that BC off and observe the natural heating of the room. Which require to use constant temperature wall for a certain period of time, and than switch it to a regular wall with no thermals.

One way would be to turn off the energy equation but then you wont get any temperature data in the entire domain. I would turn the wall into a symmetry boundary condition or make the wall adiabatic, if possible in your case.

Dear Apurv and Engin;
Defining a wall temperature (space of time dependent) was not my difficulty at the first place, which can be done DEFINE_PROFILE (as Engin suggested). My difficulty is to change the wall type by UDF (from specified temperature to just a regular wall with no thermal BC attached).

Assume that you have heater surface that you heat your room. Once your room reaches a certain temperature, and you turn it off, and the heater surface cools off naturally (and repeat the process). Must be a way to do it, but I don't know how!

Sorry for the late reply. I think you can do that but it would not be so easy. My suggestion is to use fluent scripting or GUI to iterate the solution upto the time you turn off your heat generation source (given by temperature distribution) and switch the thermal boundary condition of the wall to convection giving the temperature values of the cells near the wall using a UDF and hooking it to the freestream temperature of the flow. F_T(f,t) macro will provide you the cells temperature values if it is refered to your wall in the UDF.

It is possible to change the BC type. You can try this with two options:
1) Define the UDF and execute on demand
2) In UDF itself you suggest program to modify your BC. This will be based on your physical time.